The dynamics of intraguild predation in Chrysomya albiceps Wied. (Diptera: Calliphoridae): interactions between instars and species under different abundances of food

A dinâmica de predação intraguilda em Chrysomya albiceps Wied. (Diptera: Calliphoridae): interações entre instares e espécies sob diferentes abundâncias de alimento

ECOLOGY, BEHAVIOR AND BIONOMICS

The dynamics of intraguild predation in Chrysomya albiceps Wied. (Diptera: Calliphoridae): interactions between instars and species under different abundances of food

A dinâmica de predação intraguilda em Chrysomya albiceps Wied. (Diptera: Calliphoridae): interações entre instares e espécies sob diferentes abundâncias de alimento

Gisele S. Rosa^I; Lidia R. de Carvalho^II; Sergio F. dos Reis^III; Wesley A.C. Godoy^I

^IDepto. Parasitologia. Instituto de Biociências, Univ. Estadual Paulista, Rubião Junior 18618-000, Botucatu, SP

^IIDepto. Bioestatística. Instituto de Biociências, Univ. Estadual Paulista, Rubião Junior 18618-000, Botucatu, SP

^IIIDepto. Parasitologia, Instituto de Biologia, Univ. Estadual de Campinas, Universidade Zeferino Vaz, R. Monteiro Lobato s/ no., C. postal 6109, 13083-970, Campinas, SP

ABSTRACT

The pattern of larval interaction in blowflies confined with Chrysomya albiceps Wied. and C. rufifacies Maquart can be changed in response to the predatory behaviour of the two species to a contest-type process instead of the scramble competition that usually occurs in blowflies. Facultative predation is a frequent behaviour in C. albiceps and C. rufifacies that occurs as an alternative food source during the larval stage. In this study, we investigated the dynamics of intraguild predation by C. albiceps on other fly species in order to analyse interspecific and intraspecific survival in C. albiceps, C. megacephala and C. macellaria Fabricius. The experimental design of the study allowed us to evaluate how factors such as species, density and abundance of food influenced the survival of the calliphorid species. When C. albiceps was confined with C. megacephala or C. macellaria, only adults of C. albiceps survived at different larval densities and abundance of food. In addition, the survival of C. albiceps was higher in two-species experiments when compared to single species experiments. The implications of these results for the dynamics of C. albiceps were discussed.

Key words: Larval behavior, blowfly, competition

RESUMO

O padrão de interação larval em moscas-varejeiras na presença de Chrysomya albiceps Wied. e C. rufifacies Maquart pode ser alterado em função do comportamento predatório das duas espécies, mudando a estratégia de competição do tipo explorativa para competição por interferência. A predação facultativa é um comportamento freqüente em C. albiceps e C. rufifacies durante o estágio larval. Neste estudo, investigamos a dinâmica de predação intraguilda por C. albiceps sobre outras espécies de moscas, em experimentos delineados para analisar a sobrevivência interespecífica e intraespecífica em C. albiceps, C. megacephala Fabricius e C. macellaria Fabricius. O delineamento experimental do estudo permitiu avaliar de que modo fatores como, espécies, densidade e abundância de alimento, influenciaram a sobrevivência das espécies em culturas puras e mistas. Quando C. albiceps foi confinada com C. megacephala ou C. macellaria, somente adultos de C. albiceps sobreviveram em diferentes densidades e abundância de alimento. Além disso, a sobrevivência de C. albiceps foi superior em culturas mistas se comparada aos experimentos com culturas puras. As implicações desses resultados para a dinâmica de C. albiceps foram discutidas.

Palavras-chave: Comportamento larval, mosca-varejeira, competição

Nearly 30 years ago, three species of blowflies from Africa and Asia, Chrysomya albiceps Wiedemann, C. megacephala Fabricius and C. putoria Wiedemann, became established in the Americas (Guimarães et al. 1978). This invasion led to a sudden decline in the numbers of ecologically similar native American species, such as Cochliomyia macellaria Fabricius and Lucilia eximia Wiedemann (Prado & Guimarães 1982). C. albiceps, one of the invading species, is a carcass feeder frequently involved in secondary myiasis in sheep (Zumpt 1965).

Blowfly species frequently show different competitive abilities which, when associated with other types of behaviour, such as predation or cannibalism, interfere with coexistence by maintaining different species present in patches in spite of their population sizes, and by excluding one of them (Goodbrod & Goff 1990, Ullyett 1950). For C. albiceps and C. rufifacies Fabricius, local competition can cause the competitor to become an active predator, and can lead to a contest-type process instead of the scramble competition that usually occurs in blowflies (Ullyett 1950, Nicholson 1954). Facultative predation is a good example of the interaction that occurs as an alternative food source, and the blowflies C. albiceps and C. rufifacies shows such behavior during the larval stage (Wells & Greenberg 1992a, b, c).

Facultative predation by Chrysomya species was classified as intraguild predation (IGP) by Polis et al. (1989), who suggested that IGP can lead to faster growth and earlier metamorphosis in these organisms. Intraguild predation is usually categorized as a combination of competition and predation that involves the killing and eating of species that use similar, often limiting, resources and are thus potential competitors (Polis et al. 1989). Such predation can be distinguished from traditional concepts of competition by the immediate energetic gains for the predator, and differs from classic predation because the act reduces potential exploitative competition (Polis et al. 1989).

The specific predatory behaviour of C. albiceps has been investigated through choice and no-choice experiments designed to evaluate larval predation rates and prey choice by C. albiceps (Faria et al. 1999, Faria & Godoy 2001). C. albiceps attacks C. macellaria more often than C. megacephala and C. putoria (Faria et al. 1999). However, in the absence of C. macellaria, C. albiceps attacks C. putoria more often than C. megacephala (Faria & Godoy 2001). Attacks occur more often within the carcass, but may also occur outside, during larval dispersal (Andrade et al. 2002, Gomes & Von Zuben 2005). Grassberger et al. (2003) reported that the mortality rate of Lucilia sericata Meigen caused by predation from C. albiceps ranged from 57.6% to 99%, indicating a high susceptibility of L. sericata to attack by C. albiceps. This type of interaction may have serious implications for the faunal structure of necrophagous flies and, consequently, for forensic entomology in which the abundance and coexistence of species are important data (Gomes & Von Zuben 2005), since carcasses can be almost monopolised by a single predator fly species such as C. albiceps (Grassberger et al. 2003).

The negative influence of C. albiceps on the abundance of other blowfly species has also been intensely studied, and the results of these investigations clearly suggest that this behaviour can be an important factor in the displacement of native species in the New World (Wells & Greenberg 1992 a,b,c; Faria et al. 1999). However, biological invasions and the colonization of new areas must be evaluated not only by abundance, but also by factors such as patch habitat availability and interspecific interactions (Wells & Greenberg 1992a,b,c; Tilman & Kareiva 1997).

Despite these studies, it is not yet clear how intraguild predation occurs in situations that involve different amounts of food and densities. Important aspects that need to be considered when assessing interaction dynamics and carrying capacity include the influence of larval densities and the amount of food on predator and prey interactions, as well as the factors that determine a switching of behaviour from competitor to predator. In this study, we investigated the dynamics of intraguild predation by C. albiceps larvae on themselves and on the larvae of C. megacephala and C. macellaria in experiments designed to analyse double and single species survival. The effects of interactions among instars and of food abundance on blowfly survival were also estimated in order to understand what governs the intraguild predation by C. albiceps in a context of carrying capacity.

Material and Methods

Laboratory populations. Laboratory populations of C. albiceps, C. macellaria and C. megacephala were established from specimens collected on the campus of the Universidade Estadual Paulista, Botucatu, São Paulo, Brazil. Adult flies were maintained at 25 ± 1ºC in cages (30 cm x 30 cm x 30 cm) covered with nylon and were fed water and sugar ad libitum. Adult females were fed fresh beef liver to allow the complete development of the gonotrophic cycle. Hatched larvae of C. albiceps were reared on an excess of ground beef until the 3^rd instar when they were removed and placed in empty vials (10 cm height × 7 cm wide). These larvae were considered to be predators, since 3^rd instar is the life stage at which predation rates have been considered the highest (Wells & Greenberg 1992a, Faria et al. 1999).

Hatched larvae of C. macellaria and C. megacephala were reared as described for C. albiceps, but were only allowed to reach the 2^nd instar since they were considered prey. Strong predator-prey interactions classically imply differences in size, with predators generally being larger than prey (Faria et al. 1999, Faria & Godoy 2001). The larval instars were determined using accepted morphological characters to identify the various developmental stages (Prins 1982). One and two-species interaction experiments were set up, with 50% of larvae for each instar, i.e. 50% being 3^rd instar (predators) and 50% being 2^nd instar (prey).

The experiments were also conducted with larval densities of 200 and 1000 at the 1:1 proportion of predators (3^rd-instar larva of C. albiceps) and prey (2^nd-instar larva) . For two-species experiments, the following treatments were performed: 100 3^rd-instar larvae of C. albiceps with 100 2^nd-instar larvae of C. megacephala, 100 3^rd-instar larvae of C. albiceps with 100 2^nd-instar larvae of C. macellaria, 500 3^rd-instar larvae of C. albiceps with 500 2^nd-instar larvae of C. megacephala, and 500 3^rd-instar larvae of C. albiceps with 500 2^nd-instar larvae of C. macellaria. Each larval density was studied under three levels of food abundance, namely, no food (1), moderate food abundance (2) and high food abundance (3), which corresponded to 0, 25 and 50 g of ground beef, respectively. The survival was estimated for each larval density and treatment by recording the number of adults obtained in the experiments. Three replicates (Grassberger et al. 2003) were used for the density of 200 and five for the density of 1000 based on a previous work (Reis et al. 1999).

Statistical analysis. The results were expressed as the mean ± S. D. Statistical comparisons were performed by using a three-way ANOVA with food abundance, species and densities as the factors. The Tukey test was used for multiple comparisons. The analyses were done using the SAS software (SAS Institute 1989). Values of P < 0.05 indicated significance (Table 1).

Results and Discussion

When C. albiceps was confined with C. megacephala or with C. macellaria, the only survivors at the two densities and three treatments were adults of C. albiceps (Figs. 1 and 2). Since two-species experiments of C. albiceps completely eliminated C. megacephala and C. macellaria individuals, no statistical comparisons were done between one and two-species experiments. C. albiceps had a higher survival in double compared to single species cultures at a density of 200 for the three treatments (Fig. 3). At a density of 1000, only one case of double culture (C. albiceps × C. megacephala, treatment 2) had a lower survival than the single culture for C. albiceps (Fig. 3). The survival for C. albiceps at a density of 1000 was higher when this species was confined with C. megacephala in treatment 1, and with C. macellaria in treatments 2 and 3 (Fig. 3).

The sources of variation, degrees freedom, F- values and P-values are shown in Table 1. The difference among the survival percentages was significant for all sources of variation shown in Table 1, except for the factor species, when separately analysed. The survival percentages did not differ significantly among single species when analysed without consider the factors abundance and availability of food, which are narrowly associated with the environmental carrying capacity. Both density and different levels of abundance, expressed in this study as treatments, influenced significantly the survival percentages (Table 1). This kind of influence has been also observed in experiments focused on intraspecific larval competition, where survival rates among blowfly species were very similar, leading to identical results in terms of dynamic behaviour, investigated by mathematical models of population growth (Godoy et al.1993, Von Zuben et al. 1993).

The dynamic behaviour in insects can be understood as the temporal trajectory pattern exhibited by the population, expressed frequently by different types of oscillations, which are strongly influenced by demographic parameters, such as survival and fecundity (Prout & McChesney 1985, Godoy et al. 2001). Density-dependent mechanisms generally are associated to variation of demographic values, and consequently of dynamic behaviour in blowflies (Godoy et al. 1996). In our study, both the three way ANOVA and the Tukey test for multiple comparisons showed that the influence of the factorial interaction on the survival percentages is significant, indicating that treatment, densities and species are factors capable to change the demograpic values in blowflies, with probable consequences for the species population dynamics.

The results suggested that, intraguild predation by C. albiceps occurred in the interactions, since no larvae of C. megacephala or C. macellaria were found after the confinements. Theoretical studies have shown that intraguild predation can affect population dynamics because the intraguild prey have difficulty to survive in food webs where they compete for food and are subjected to predation (Holt & Polis 1997, McCann & Hastings 1997). Based on an isocline analysis of an intraguild predation model, Polis et al. (1989) showed that if two species coexist without intraguild predation then adding intraguild predation can lead to exclusion of the intraguild prey from the system. Reis et al. (1999) observed that C. putoria and C. macellaria coexisted where there is no C. albiceps larvae. Both intraguild predators and prey can coexist only if the intraguild predators are less inferior than the intraguild prey in exploiting a common resource (Holt & Polis 1997). Moreover, if the intraguild predator is an efficient competitor for shared resources then, even without intraguild predation, the intraguild prey will be eliminated. Holt & Polis (1997) also argued that if intraguild predators follow optimal foraging theory rules, then coexistence might be achieved by dropping the intraguild prey from the predator's diet when food is abundant.

Intraguild predation by C. albiceps and C. rufifacies has also been observed on other fly species (Ullyett 1950; Wells & Greenberg 1992 a,b,c). In this study, C. albiceps showed higher survival in most of the two-species experiments, suggesting that predation offers more advantages to C. albiceps than competition for food and cannibalism (Faria et al. 1999, 2004). However, interspecific competition might be more advantageous than intraspecific competition, with the preference generally depending on the strength of the interactions between the species (Reis et al. 1999).

Intraguild predation provides an alternative resource, when food is scarce, resulting in a decrease of competition for food if one of the competitor species also acts as a predator (Hanski 1981, Polis et al. 1989). Many predators are also cannibals, including C. albiceps (Faria et al. 2004), and the offspring size, growth rate, longevity and reproductive phenology may influence the strength and direction of intraguild predation (Reaka 1987). Intraguild predators can benefit from reduced competition, especially competition for local resources (Mabelis 1984). In some cases, intraguild predation is sufficiently severe to reduce or eliminate the prey population, including insects that feed on carcasses (Polis et al. 1989).

At the initial stages of confinement, larvae of both C. megacephala and C. macellaria avoid interaction with C. albiceps larvae by migrating to the vial walls or penetrating the food substrate (Rosa et al. 2004). Population theory suggests that a stable coexistence amongst competing species is only possible if the species are sufficiently ecologically distinct, i.e. they must have different roles in order to live in the same community (Zhang & Hanski 1998).

Based on the results of this study, we conclude that the three factors analysed here (species, amount of food and density) were all important for the interactions amongst blowflies. Competition, predation and cannibalism in experimental and natural populations are interactions that, in association with food abundance, may produce a variety of effects on the natural pattern of interactions in necrophagous dipteran communities, in order to understand what governs the intraguild predation and the population dynamics of these groups.

Acknowledgments

The authors thank Dr. S Hyslop for criticism and suggestions and also for reviewing the English of the manuscript. Two anonymous reviewers provided valuable insight and offered several helpful suggestions for improving an earlier version of this article. GS Rosa was supported by fellowships from CAPES, WACG and SFR by partial support from CNPq. This research was supported by grants from FAPESP (01/11235-1).

Received 04/03/05.

Accepted 31/03/06.

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Publication Dates

• Publication in this collection
  31 Jan 2007
• Date of issue
  Dec 2006

History

• Received
  04 Mar 2005
• Accepted
  31 Mar 2006